Glass Processing Lecture #17. Silica Glass Processing Part 1 Thomas P. Seward III Corning Incorporated (retired) Alfred University (retired) ([email protected])
Spring 2015
Lectures available at: www.lehigh.edu/imi Sponsored by US National Science Foundation (DMR-0844014)
1 IMI-NFG’s Glass Processing course, Spring 2015. Outline
Day One
1. Introduction - Silica Glass (SiO2) - Its technological importance (Properties, Applications) – Its manufacturing difficulties 2. Innovations in manufacturing history 3. Type classifications for transparent silica glass 4. Silica glass from natural raw materials Processes and resulting characteristics Day Two 1. Synthetic fused silica manufacturing Processes and resulting characteristics Deliberate addition of modifying chemical species. 2. Modern Applications Examples -Telescope mirrors, microlithography optics, optical fiber 3. Summary and Acknowledgements
2 IMI-NFG’s Glass Processing course, Spring 2015. A Unique Material
Silica glass is a quite unique material. It is the only single-oxide glass former that is widely used on its own (not having its chemical composition highly modified). It has properties unlike any other material. These properties are in demand for many applications, some of them high-tech. Or at least essential to manufacturing high-tech products. Let’s look at come of these properties.
3 IMI-NFG’s Glass Processing course, Spring 2015. Fused Silica Characteristics
Low thermal expansion (0 to 300 ºC) about 0.55 ppm/ºC or 5.5 x 10-7/ºC High thermal shock resistance
Refractory – High viscosity and high use temperatures Viscosity about 107 poise at 1700˚C Softening point generally above 1550 ˚C)
High optical transparency; wide transmission band (DUV- near IR) Excellent chemical durability
Good radiation damage resistance
High electrical resistivity, low dielectric constant and low loss tangent.
4 IMI-NFG’s Glass Processing course, Spring 2015. Fused Silica Applications
Key properties: Excellent optical transparency, low thermal expansion, high use temperature, low electrical conductivity, low alkali content,
Manufacturers’ Product Literature
5 IMI-NFG’s Glass Processing course, Spring 2015. Important Fused Silica Applications
Because of silica’s almost unique combination of characteristics, it is used for many different specialized applications: – Lighting (high-intensity-discharge, quartz-halogen, and uv lamp envelopes) – Semiconductor industry manufacturing (crucibles, furnace tubes, substrates, microlithography optics) – Optical elements (lenses, windows, mirrors) including high energy laser optics – Optical fiber and photonic devices – Astronomical telescope mirrors – Space craft windows – Other (including labware, tubing, specialty fiber and wool-type fiberglass).
6 IMI-NFG’s Glass Processing course, Spring 2015. Group Applications by Property
Refractory – High temperature lamp envelopes – Spacecraft windows – Crucibles and furnace refractories High optical transparency – UV optics – Optical fiber for telecommunications Low thermal expansion – High temperature lamp envelopes – Spacecraft windows – Thermally stable mirrors (astronomical telescopes, IC microlithography stepper camera optics) High purity – IC microlithography stepper camera optics
7 IMI-NFG’s Glass Processing course, Spring 2015. Available in a Variety of Forms
Powders Coatings Thin films Fibers Porous bodies Bulk glass – Translucent – Transparent
This and the following lecture will concentrate primarily on bulk transparent glasses.
8 IMI-NFG’s Glass Processing course, Spring 2015. Nomenclature
Commonly used names: - Silica glass (but sometimes this name is used for all silicate glasses) - Vitreous silica - Fused quartz, quartz glass, and sometimes just “quartz” (sometimes reserved for when natural quartz or rock crystal is the raw material) - Fused silica (at one time only used when sand was the raw material) - Synthetic fused silica, synthetic quartz, synthetic quartz glass (when made from synthetic raw materials such as silicon tetracloride and siloxanes)
Frequently all but the last are used interchangeably.
9 IMI-NFG’s Glass Processing course, Spring 2015. Intrinsic vs. Extrinsic Behavior
Transport properties (e.g. viscosity and electrical conductivity) are very dependent on purity and manufacturing method through:
- water content (H20)
- impurity content (other than H20) - thermal and/or manufacturing history The intrinsic capabilities of silica may never have been measured.
From Encyclopedia of Chemical Technology, 1994
10 IMI-NFG’s Glass Processing course, Spring 2015. Properties Affected by Purity
Two Examples
Viscosity Near Infrared Absorption
11 IMI-NFG’s Glass Processing course, Spring 2015. Silica Glass and Liquid Structure
The basic building block of all condensed phases of silica (liquid, crystal or glass) is
the SiO4 tetrahedron.
In the liquid or glass states, these tetrahedra are bonded together through shared oxygens to make a continuous structure.
One may ask, if all the tetrahedra are bonded to each other, how can the liquid flow? Or, how does the solid glass soften when heated?
Good questions!
12 IMI-NFG’s Glass Processing course, Spring 2015. Viscous Flow of Silica
For flow, broken bonds must be present. Thermal energy tends to break bonds. With increasing temperature, progessively more bonds are broken. The more broken bonds, the more easily the glass flows (viscosity decreases).
13 IMI-NFG’s Glass Processing course, Spring 2015. Effect of Impurities/Modifiers on Viscosity
Other factors can provide broken bonds.
For example, each molecule of soda (sodium oxide) added creates one NBO (non-bridging oxygen).
Other modifiers have a similar effect. For example, H2O.
It makes sense that the more modifiers present, the more easily the glass flows.
14 IMI-NFG’s Glass Processing course, Spring 2015. Effect of Dissolved H2O on IR Transmission
Corning Code 7980 Type III 800-1000 ppm OH
Corning Code 7979 Type IV < 1 ppm OH
Internal Transmittance (corrected for reflection losses)
15 IMI-NFG’s Glass Processing course, Spring 2015. Key Point So Far
Important properties of silica glass depend on its degree of purity. Impurities include – metal oxides, other glass-forming oxides, dissolved gases (including hydrogen, chlorine, and H2O). Purity in turn depends on the starting raw materials (chemical sources) and the manufacturing process. The ultimate properties of “absolutely” pure fused silica have never been measured.
We will look at how raw materials and processes produce subtle, and sometimes not so subtle, variations in properties.
The processes themselves are generally expensive and so are the products.
16 IMI-NFG’s Glass Processing course, Spring 2015. Exercise
We just saw examples of how impurities such as water and alkali oxides can affect viscosity and optical transmittance of silica glass.
Think of some other properties of silica glass and how different impurities might or might not affect them.
Note: This exercise and others to follow are not for class discussion, but could be considered as useful homework.
17 IMI-NFG’s Glass Processing course, Spring 2015. Manufacturing Processes
18 IMI-NFG’s Glass Processing course, Spring 2015. Modern Processes
We will see that all modern process for manufacturing bulk silica glass involve the preparation and viscous sintering of small, amorphous (glassy) silica particles.
How this viscous sintering is accomplished, effects the purity and homogeneity, both chemical and structural.
Variables including the size and composition of the starting particles and the method used for sintering.
19 IMI-NFG’s Glass Processing course, Spring 2015. Raw Materials and Heat Sources
The starting materials are primarily of two types – Crystalline quartz of varying purity from natural quartz deposits (in places like Brazil and Madagascar) or high quality glass makers sand (for example from Pennsylvania and West Virginia) – Liquid compounds, both inorganic and organic, e.g., halides, siloxanes, and teraethylorthosilicate (TEOS)
The heat sources also are primarily of two types – Electrical – electric arcs, resistance and induction heaters, and high frequency plasma discharges – Chemical combustion – hydrogen-oxygen and natural gas- oxygen burner flames (Natural gas is primarily methane.)
20 IMI-NFG’s Glass Processing course, Spring 2015. Crystalline Forms of Silica
Stability ranges: Cristobalite 1470 - 1723 ºC Trydamite 870 - 1470 ºC Quartz below 870 ºC Low quartz to high quartz phase transition at 573 ºC
On heating, quartz transforms to cristobalite before it melts T > 1730 ºC required to form an all-liquid melt
Boiling point 2300 ºC
21 IMI-NFG’s Glass Processing course, Spring 2015. Viscosity – Temperature Relationships Different glass compositions can have dramatically different temperature dependences.
– 0080 is a soda-lime-silica glass. – 7740 is original Pyrex – 1720 is an aluminosilicate glass.
Orange indicates viscosities needed for batch melting and fining in traditional glass furnaces.
Silica is way off-scale!
22 IMI-NFG’s Glass Processing course, Spring 2015. Manufacturing Challenges
The refractoriness poses major melting difficulties – The high temperature crystalline phase (cristobalite) melts at about 1725 ºC. – The viscosity at that temperature is about 107 poise. – This rules out conventional melting techniques.
Silica melts tend to react with their containers. For example
– SiO2 + C SiO (gas) + CO (gas)
At high temperatures SiO2 is noticeably volatile. The boiling point is only 2,300 ºC.
The manufacturing processes often introduce chemical impurities, including dissolved water and halogen gases, but high chemical purity (50-100 ppb impurities) is often required by applications.
23 IMI-NFG’s Glass Processing course, Spring 2015. Effect of Melting Atmosphere
During glass melting processes, molten glass tends toward chemical equilibrium (to varying degrees) with the furnace atmosphere.
Some atmospheric gases dissolve into the glass changing its properties.
Example gases: Cl2, CO2, H2O
This is true when processing SiO2 as well as when processing more traditional glasses.
24 IMI-NFG’s Glass Processing course, Spring 2015. Effect of Furnace Atmosphere on Glass Composition Electric furnace
– Air approx. 80% N2 and 20% O2, a relatively dry, oxidizing environment Gas-air fired furnace
– Lots of N2 from the air, lots of the combustion products CO2 and H2O, and probably some O2, depending on how the burners are adjusted. Gas-oxygen fired furnace
– Little N2, so mostly CO2, H2O and some O2, depending on how burners are adjusted. If the gas is H2, then little CO2 produced. Effects: – Electric furnaces tend to be more oxidizing and introduce little
H2O. – Glass made in gas-oxy furnaces tend to contain a higher
concentration of H2O (1,000 ppm or greater). 25 IMI-NFG’s Glass Processing course, Spring 2015. Manufacturing Objectives
First, find or develop a method for melting or otherwise making good quality transparent pieces of glass of a size appropriate to the application.
Second, make the glass of sufficient purity and homogeneity to meet the requirements of the application.
With silica these are difficult objectives. To achieve them is relatively expensive.
26 IMI-NFG’s Glass Processing course, Spring 2015. Early Processing History
27 IMI-NFG’s Glass Processing course, Spring 2015. Innovation Pre-20th Century
19th century - introduction of oxygen-injected or oxygen/fuel (as opposed to air/fuel) burners and torches allowed sufficient energy concentrations to melt crystalline quartz (> 1725 ºC); also
lampworking with H2-O2 flame (including capillaries, bulbs and drawn fiber). Some dates
– 1813 - First recorded melting of small quartz crystals with O2 injected alcohol burner
– 1821 – Melting of quartz crystals and lampworking wit H2/O2 torch – 1887 - Drawn fiber (cross-bow technique) – 1878 – Lamp-worked capillaries and thermometer bulbs at the Paris Exhibition
28 IMI-NFG’s Glass Processing course, Spring 2015. Innovation in the Early 20th Century
1899-1910 - commercial development in England, France and Germany - key issues were purity and transparency – production generally limited to translucent and semi-transparent crucibles, fiber, rods and tubing.
Opaque materials called “fused silica” made by fusing sand with carbon arcs or resistively heated carbon electrodes Two examples:
Carbon Arc Carbon Resistance Heating
Hutton - 1901 Bottomly, Hutton and Paget - 1904
29 IMI-NFG’s Glass Processing course, Spring 2015. Early 20th Century continued
Tubing was sometimes made by drawing cane from a molten blob, then wrapping around a platinum rod and re-fusing.
Transparent materials were made by melting clear, selected quartz crystals in an arc or flame and collecting them on a heated rod or surface below. – Arc Fusion » Shenstone and Kent, 1903 – Flame Fusion » W.C. Heraeus, 1908
These processes form the bases of modern technology and we will look at them further, but essentially they provide melts that form and solidify with the only solid contact being the glass that was previously formed. They are containerless process.
30 IMI-NFG’s Glass Processing course, Spring 2015. Innovation Mid 20th Century
The beginnings of the modern era. This is where the processing part of the course really begins:
Electric melting and flame fusion were further developed. Continuous drawing of rod and tubing from vertical electric furnaces became possible. Induction heating was introduced.
Synthetic processes, in particular flame hydrolysis (1934) - U.S. patent by J.F. Hyde (Corning Glass Works) on flame hydrolysis of
SiCl4 issued in 1942; the basis for all synthetic fused silica processes used commercially today.
31 IMI-NFG’s Glass Processing course, Spring 2015. Manufacturers
There are a great many (not a complete list) America – General Electric Heraeus Amersil – Corning Incorporated Thermal American Fused Quartz – Osram Sylvania Dynasil Corp. of America (Russia) Germany – Heraeus Quarzschmelze France – Quartz et Silice England – Thermal Syndicate Ltd./Saint Gobain Japan – Toshiba Ceramics – Shin-Etsu – Tosoh./NSG
32 IMI-NFG’s Glass Processing course, Spring 2015. Transparent Silica Types
From K-O Encyclopedia of Chemical Technology, 1994
33 IMI-NFG’s Glass Processing course, Spring 2015. Transparent Fused Silica Types
Chart to Assist Memory
Electric Flame Melting Fusion
Natural Type I Type II Less Quartz Pure
Synthetic Type IV Type III More Precursors Pure
Dry Wet
34 IMI-NFG’s Glass Processing course, Spring 2015. Exercise
We have talked about how the high melting point of cristobalite (the most refractory crystalline form of silica) and the high viscosity of the melt at that temperature makes traditional glass melting methods impractical or impossible to use for silica.
Considering what you already know about traditional glass forming processes, which of them do you think could or could not be applied to silica glass? Why or why not?
35 IMI-NFG’s Glass Processing course, Spring 2015. Electric Melting - Type I
36 IMI-NFG’s Glass Processing course, Spring 2015. Electric Melting of Quartz – Type 1
Raw material – Source -traditionally crystalline quartz , the most pure being from Brazil and Madagascar – More recently, beneficiated high quality glassmaking sand – Multi-step preparation to purify and control grain size of both. Heat sources: – Electric arcs, resistance heaters or induction heaters Processes used extensively for the manufacture of crucibles, rod and tubing and objects made from them.
37 IMI-NFG’s Glass Processing course, Spring 2015. Vacuum and Pressure
A major ongoing obstacle to optical clarity was the trapping of air between the quartz grains as they were melted and sintered together. Osram experimented with vacuum melting before WWII. General Electric pursued melting under vacuum before 1930 to remove the trapped air, then further consolidating the sintered mass under high pressure to minimize any remaining voids. The resulting mass of glass was called an ingot. Ingots could be further worked at high temperatures to produce many shapes. GE made telescope mirror blanks using this process by creating hexagonal shapes (6.5” and 22”) and fusing them together in three layers. The one made for Kitt Peak in 1967 was 158” diameter. (This “hex-seal” process will be discussed further next week in conjunction with synthetic silica manufacturing.)
38 IMI-NFG’s Glass Processing course, Spring 2015. Electric Lighting Spurred Development
High pressure mercury arc introduced in 1934.
Necessitated high volume manufacture of silica glass tubing for arc chamber.
Lamp manufacturers lead the process development.
Heinlein (1939) is credited with the first commercial continuous process process.
Essentially a cold-crown, electrically heated vertical furnace with tubing drawn from an orifice at the bottom.
Many refinements since then.
General Electric Product Literature
39 IMI-NFG’s Glass Processing course, Spring 2015. Heinlein (Osram) Patent - 1939
• Continuous drawing of tubing or rod • Tungsten filament heaters (m.p. 3,400 ºC) • Crucible, die (tapered orifice) and hollow mandrel made of molybdenum (m.p. 2,600 ºC) • Non-oxidizing atmosphere, e.g. nitrogen or forming gas (nitrogen/hydrogen mix) • Crushed quartz fed in at the top, tubing pulled by rollers and belts below.
40 IMI-NFG’s Glass Processing course, Spring 2015. Variations
Improvements and/or variations on the OSRAM process include: – Alternative refractory materials as insulation and for the glass contact materials – Replacing resistance heating with induction melting, which involves electromagnetically coupling the elecrical energy to the crucible which acts as a “susceptor.”
41 IMI-NFG’s Glass Processing course, Spring 2015. Type I Tubing Applications
Examples: – Electric lighting
– Semiconductor (Integrated circuit) processing: rods, tubing, wafer carriers
– Optical fiber substrate tubing (for the OVD/ MCVD process we will introduce next week).
General Electric Product Literature
42 IMI-NFG’s Glass Processing course, Spring 2015. Electric Arc Fusion – Type I
While technically not considered Type I because they are not transparent, translucent crucibles made by electric arc fusion qualify in terms of their purity levels and method of manufacture.
They are an extremely important product used in many laboratory and commercial processes.
GE 500 Series “Quartz” Crucibles General Electric Product Literature
43 IMI-NFG’s Glass Processing course, Spring 2015. Summary - Silica Glass - Type I
– Traditionally called “fused quartz” or just “quartz” – Produced from natural quartz, sand, or beneficiated sand – Electric fusion under vacuum or inert gas – “Dry” - contains < 200 ppm OH (water) – Some metallic impurities (e.g. < 200 ppm Al; < 400 ppm transition metals; < 50 ppm Na, but can be much less) – depends on the purity of the raw materials – Examples: – Infrasil (Heraeus) – IR-Vitreosil (TSL) – General Electric 124, 214 standard lamp tubing & 224 low alkali (G.E.)
44 IMI-NFG’s Glass Processing course, Spring 2015. Flame Fusion – Type II
45 IMI-NFG’s Glass Processing course, Spring 2015. Flame Fusion of Quartz – Type II
A direct outgrowth of Heraeus’ 1908 innovation Essentially continuous deposition of molten granules of quartz or quartz sand onto a hot rotating mandrel where the granules sinter into a dense ingot or “boule” of glass. Chlorine gas can be introduced during the “laydown” of the glass to improve purity. Chlorine removes metal contaminants such as Al, Fe, Cu, Zn, Ti and the alkali and alkali-earth metals by forming volatile chlorides. Similarity to the Vernueil process for growing oxide crystals (1902), is shown on the next slide.
46 IMI-NFG’s Glass Processing course, Spring 2015. Vernueil Process
Originally developed for Silica is an excellent growing synthetic glass former. gemstones based on high melting point crystals When molten quartz such as alumina (m.p. droplets settle on a 2072 ºC ). Examples: substrate above about ruby and sapphire. 1600 ºC, they fuse together into a molten mass. Crystalline powder melted in oxy-hydrogen flame. This mass does not crystallize at those temperatures nor Droplets land on seed when being cooled to crystal and crystallize in room temperature. same orientation as seed. The resulting mass of The crystal mass is called glass is also called a a “boule.” “boule.”
47 IMI-NFG’s Glass Processing course, Spring 2015. Quartz Preparation
Select highest purity raw material available Treat before melting to improve purity and reduce formation of gaseous inclusion during fusion – Wash in mixed acids to remove surface contaminants. – Heat to > 800 ºC to cause micro-cracking from the from the high- low quarts transition. – Plunge into distilled water to enhance micro cracking. – Dry. Such powders are suitable for vacuum sintering as well as flame fusion, because they – Melt faster and – Release gaseous impurities more quickly
48 IMI-NFG’s Glass Processing course, Spring 2015. Heraeus Flame Fusion (Example)
From “Fused Quartz Manufacture in Germany,” 1946
49 IMI-NFG’s Glass Processing course, Spring 2015. Heraeus Burner Detail
From “Fused Quartz Manufacture in Germany,” 1946
50 IMI-NFG’s Glass Processing course, Spring 2015. Flame Fusion on a Large Scale
Pursued by General Electric Large slabs of fused silica (up to 6 feet) for telescope mirrors were generated by traversing the oxy-hydrogen torch, with granular quartz injected, across the slab, gradually building the thickness layer by layer. Major achievement was the prototype of a 200 inch telescope mirror for the Mount Palomar mirror. GE lost out to Corning Glass Works’ cast borosilicate mirror because of cost and manufacturing difficulties.
51 IMI-NFG’s Glass Processing course, Spring 2015. Summary - Silica Glass - Type II
– Also called “fused quartz” – Produced from quartz crystal powder – Oxy-hydrogen flame fusion (similar to Verneuille crystal growth) – “Wet” - 150 to 400 ppm OH from combustion products – Less metallic impurities than Type I – Alkali < 5 ppm; Transition metal < 10 ppm; Aluminum 10-50 ppm – Examples: – Homosil & Optosil (Heraeus) – OG Vitreosil (TSL) – General Electric 104 (G.E.)
52 IMI-NFG’s Glass Processing course, Spring 2015. Lampworking Considerations
Because of high viscosity and volatility at high temperatures, lampworking is more challenging than with more common soda- lime-silica and borosilicate glasses. Natural gas/oxygen, hydrogen/oxygen and acetylene/oxygen type torches used to reach temperatures of 1700 – 1900 ºC. Rapid cooling outside the flame necessitates working within the flame at all times. Only minor annealing at about 1050 ºC is required. Cleanliness is essential to avoid surface contaminants leading to devitrification while working the glass.
Volatilized SiO2 can redeposit on cooler regions producing a white powdery appearance (bloom). This can be removed by heat or dilute HF acid.
53 IMI-NFG’s Glass Processing course, Spring 2015. Electric Arc Fusion for Refractories
Quartz sand is melted in an electric arc furnace at temperatures > 2000 ºC. Resulting glass ingots are crushed and ground to size suitable for injection molding or slip casting. Shaped articles are dried and sintered (fired) at 1150-1250 ºC. Typical products are white, opaque refractory bricks, crucibles, nozzles and pouring Heroult-type electric arc furnace. tubes used elsewhere in the Illustration only. Not intended to glass industry. They are called represent geometry of actual quartz fused silica refractories. melting furnace. Not a focus of these lectures.
54 IMI-NFG’s Glass Processing course, Spring 2015.
Exercise
From what we have covered today, explain why the amount of water dissolved in silica glass differs between Type I and Type II. Which one is greater? Why? Would you expect that trend always to be the case? Why or why not?
55 IMI-NFG’s Glass Processing course, Spring 2015. Next Week
Day Two 1. Synthetic fused silica manufacturing Processes and resulting characteristics Deliberate addition of modifying chemical species.
2. Modern Applications Examples -Telescope mirrors, microlithography optics, optical fiber
3. Summary and Acknowledgements
56 IMI-NFG’s Glass Processing course, Spring 2015. Acknowledgements
Most of the illustrations are from manufacturers product brochures, the patent literature or technical literature.
Useful references include – Various editions of The Encyclopedia of Chemical Technology » W. Winship; W.H. Dumbaugh and P.C. Schultz; P.S. Danielson; D.R. Sempolinski and P.M. Schermerhorn – Commercial Glasses (Adv. in Ceramics 18) » P. P. Bihuniak – “Fused Quartz Manufacture in Germany,” Final Report No. 1202, British Intelligence Objectives Sub-Committee, 1946. – See suggested reading list for details.
57 IMI-NFG’s Glass Processing course, Spring 2015.